When two communications systems use adjacent segments of frequency spectrum, these stations may interfere with one another due to unwanted emissions or receiver overload. Depending on whether the systems involved are utilizing frequency division duplex (FDD) protocols, time division duplex (TDD) protocols, or combinations thereof, there are up to four (4) interference scenarios that are possible, i.e., base station (BS) interfering with mobile stations (MS), BS interfering with BS, MS interfering with BS, and MS interfering with MS.
As seen in FIG. 1A, there are two types of FDD systems whose systems are separate in frequency separated by some duplex frequency separation gap. First, the FDD A and FDD B system show up-link (UL) interference where a MS of FDD A interferences with a BS of FDD B. Similarly in the down-link (DL), the base station of FDD A may interfere with a mobile station in FDD B, and the base station of FDD B may interfere with a mobile station on FDD A.
As shown in FIG. 1B, when the frequency gap between the uplink of FDD A and the downlink of FDD B is relatively small, the base station of FDD B may interfere with the base station of FDD A, and the mobile station of FDD A may interfere with the mobile station of FDD B. Similarly in FIG. 1C, an interference example is shown using a combination of an FDD system and a TDD system where the TDD system transmits UL and DL data on the same frequency. In this example, on the UL side, each of the four different interference scenarios are shown, where a mobile station of FDD A interferes with the TDD base station or a TDD mobile station. Also, the FDD base station may be interfered with by the TDD base station or a TDD mobile station. Similarly, on the DL side, the FDD base station can interfere with the TDD base station or TDD mobile station. Also, an FDD mobile station can be interfered with by the TDD base station or mobile station.
Typically among these four interference scenarios, the MS-to-MS scenario tends to be the most difficult to solve due to the cost, size and weight constraints of the mobile station. Additionally, the mobile stations are at relatively random locations where signal strength between devices is continually changing. One method of minimizing such interference between devices involves synchronizing the two TDD systems and/or separating the two FDD systems in frequency. However, interference can still be problematic in cases where an FDD system is in proximity to a TDD system or the frequency gap between the uplink of one FDD system and the downlink of another FDD system is relatively small. Thus, a solution for solving the MS-to-MS interference problem is needed.
There are several ways to mitigate MS-to-MS interference. One technique involves using filters in both the transmit and receive chains of the mobile device. A disadvantage to this technique is that it increases cost, size, and weight of the mobile device. Another common approach is to utilize a “guard” band or frequency gap between the systems. Generally, this is a static approach as the width of the guard band is determined before the deployment of the systems. The main drawback of this approach is that the guard band size may be large if traditional separation criteria are used. For example, if two Worldwide Interoperability for Microwave Access (WiMAX) systems are operating on adjacent spectrum blocks, typically a 1 dB desensitization criteria for two mobile stations separated by 1 meter could require in excess of 15 MHz of guard band. Thus, this technique can require the non-use of valuable frequency spectrum which often is not a viable alternative.
Additionally, for two coexisting TDD systems, time synchronization can be employed to reduce the number of interference scenarios. If the frame duration and uplink/downlink split are synchronized, there will be no base station to base station interference or mobile station to mobile station interference. Cognitive radio concepts can also be used to mitigate the mobile station-to-mobile station interference. In general, cognitive systems are able to adapt their modulation, power and frequency to enable spectrum sharing with other systems. This may involve the inter-system communication to exchange signaling or share information (i.e., spectrum usage information) in a coordinated manner. Intersystem communication is used in the IEEE 802.16(h) standard for enabling coexistence of license-exempt IEEE 802.16 systems. Those skilled in the art will recognize that it would be ideal if an MS could differentiate high interference from deep channel fading, which impact the received signal-to-noise ratio (SNR). If the MS could check its received signal power as well as its received SNR; and if the received signal strength becomes lower, this might be due to channel fading. If the received the signal strength maintains consistent while received SNR becomes lower, this might be due to high amounts of adjacent interference.
Also, time-sharing techniques have also been used to enable system coexistence between two or more communication systems. These shared techniques use a shared broadcast control channel for multiple radio systems to share the same frequency band without causing interference to one another. In the shared broadcast channel, each system in turn broadcasts its information, such as carrier frequency, bandwidth, duty cycle, transmit power level, and the like. In this way, when one system is active, other systems refrain from transmission. An ad hoc control protocol was invented to enable different types of radio communication devices to communicate spectrum usage information to one another using a common signaling format in order to achieve harmonious sharing of an unlicensed or shared radio spectrum.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.